The present invention relates to a fluid flow device which permits electrically conductive fluid to flow therethrough from an inlet line to an outlet line, while maintaining a relatively high impedence electrical path between the inlet and outlet lines. More particularly, the present invention relates to a device forming a part of the ink return system of an ink jet printer for returning electrically conductive ink from a catcher, maintained at a high electrical potential, to a grounded fluid supply container.
In ink jet printers, such as disclosed in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis, a plurality of jet drop streams of drops of electrically conductive ink are directed toward a print receiving medium. The streams are produced by supplying ink under pressure to an electrically grounded print head which includes an orifice plate defining a plurality of orifices. Fluid filaments emerge from the orifices and are mechanically stimulated such that drops of substantially uniform size and spacing are formed from the tip of each of the fluid filaments. Charge electrodes are positioned adjacent the fluid filament tips and charge potentials are supplied to the electrodes, inducing corresponding charges of opposite polarity in the tips of the fluid filaments. The induced charges are carried away by drops which are formed from the fluid filaments.
The selectively charged drop streams thereafter pass through a deflection field extending between an electrically conductive deflection electrode, raised to a relatively high electrical potential, and a pair of grounded drop catchers. Drops in the jet drop streams which carry a charge are deflected to strike one of the drop catchers on a face thereof. The drops are thereafter ingested into the catcher through a slot extending along the bottom of the catcher face. A vacuum line is connected to an internal catcher cavity to carry away the ink ingested into the catcher cavity and to return the ink to an ink supply tank. Ink from the tank is supplied under pressure to the print head. The uncharged jet drop streams pass unaffected through the deflection field and strike the print receiving medium so as to form collectively a print image thereon.
Since the catchers of the Mathis printer are electrically grounded, electrically conductive ink from the catchers may be returned to the grounded ink supply tank and, subsequently, to the grounded print head without maintaining electrical isolation between these printer elements. When a printer element, such as the porous deflection electrode disclosed in U.S. Pat. No. 4,031,563, issued June 21, 1977, to Paranjpe et al., is maintained at an elevated electrical potential and ingests electrically conductive ink, however, it is necessary to provide some means for electrically isolating the printer element from the ink supply. In the Paranjpe et al printer, a fluid trap is provided in the vacuum line connected to the deflection electrode to ensure that the deflection electrode is electrically isolated from the rest of the recording head. The trap consists of a stoppered beaker which accumulates ink in the bottom thereof and has a vacuum line connected through the stopper to supply a partial vacuum to the air space in the beaker above the accumulated ink. The Paranjpe et al device, however, makes no provision for return of accumulated ink to the printer ink supply.
U.S. Pat. No. 3,798,656, issued Mar. 19, 1974, to Lowy et al., discloses a printer in which drop catchers, maintained at both positive and negative potentials, create deflection fields for deflecting jet drop streams to selected print positions. The catchers also catch and ingest drops which are not to be deposited upon the print receiving medium. The catchers which are held at a positive deflection potential are connected to a common vacuum manifold which carries the fluid ingested into the catchers to a denebulization chamber through which vacuum is supplied to the catchers. Similarly, the catchers which are held at a negative deflection potential are connected to a second common vacuum manifold which supplies the fluid ingested thereby to a second denebulization chamber.
Each denebulization chamber defines a cavity in which is positioned a high surface tension material, such as metal wool. Ink drops fall from the wool, through a funnel-shaped partition, into a lower portion of the cavity with the drops striking a grounded conductive plate and thereafter passing through an outlet conduit to an ink supply tank. The conversion of the stream of ink from the manifold into separated ink drops produces a high impedence path to the grounded conductive plate with the result that the catchers are not shorted to ground and may be maintained at the desired deflection potentials. The denebulization devices of the Lowy et al printer have limited fluid flow rate capabilities. Additionally, since the charged drops strike a grounded conductive plate, electrochemical degradation of the plate may occur.
U.S. Pat. No. 3,916,421, issued Oct. 28, 1975, to Hertz, and U.S. Pat. No. 4,004,513, issued Jan. 25, 1977, to Watanabe et al. both disclose other types of ink jet printers in which ink is collected on a charged drop collection surface and thereafter drips into a collection pan. The single drop stream produced in the Hertz and Watanabe et al. printers provides a high impedence path to the collection pan. Such arrangements, however, are somewhat limited in the maximum flow rate of ink which can be collected.
It is seen, therefore, that there is a need for a charge decoupling device for use in the vacuum return line of an ink jet printer for providing a high impedence electrical path from the printer catcher or other printer element, maintained at a high electrical potential, and a grounded fluid supply, while at the same time permitting a high flow rate of electrically conductive fluid therethrough.